Abstract: High temperature stress (HTS) is one of the major limiting factors that affect the quality of intensively cultured seedlings in protected facilitates during hot season. Increasing the cross adaptive response of plant induced by temperature stress is an effective way to improve plant stress resistance. In order to explore the alleviating effect of coldshock intensity on tomato seedlings under HTS, tomato seedlings were subjected to cold-shock treatments every day with 5 ℃, 10 ℃, and 15 ℃ for 10 min, 20 min, and 30 min, respectively, in an artificial climate chamber. The effect of single appropriate cold-shock on the gene expression of small heat shock proteins LeHSP23.8 and CaHSP18 was investigated. The results showed that hypocotyl elongation and plant height of tomato seedlings were restrained by coldshock treatment before HTS was met. The alleviating effect of tomato seedlings under HTS by coldshock varied greatly with levels and durations of temperature. The membrane lipids in the leaf of tomato seedlings were subjected to peroxidation injury in the cold-shock treatment at 5 ℃, in which the penetration of cell membrane was increased and the activities of antioxidant enzyme was inhibited. The alleviating effect to HTS by cold-shock was decreased with the increasing cold-shock duration at 10 ℃, however, a reverse change was found at 15 ℃. The results indicated that cross adaptive response of tomato seedling could be induced with a moderate cold-shock temperature for a proper duration before HTS was met. The optimum cold-shock treatment was at 10 ℃ for 10 min per day, under which, the dry mass, healthy index, activities of protective enzymes (including SOD, POD and CAT) in leaves of tomato seedlings were significantly increased, the contents of proline and soluble protein were enhanced, relative conductivity and malondialdehyde concentration were significantly decreased, and the expression levels of LeHSP23.8 and CaHSP18 were increased compared with that under normal temperature condition.